This invention relates in general to an electroforming mandrel and a process for utilizing the mandrel to prepare hollow electroformed metal articles.
The fabrication of hollow metal articles by an electroforming process is well known. For example, hollow metal articles are fabricated by electrodepositing a metal onto an elongated mandrel which is suspended in an electroyltic bath. The resulting seamless electroformed tubes are thereafter removed from the mandrel by sliding the tube off one end of the mandrel. Different techniques have been developed for forming and removing tubes from electroforming mandrels depending upon the cross-sectional area of the electroformed tube. Examples of these techniques are described, for example, in U.S. Pat. No. 3,844,906 to R. E. Bailey et al and in U.S. Pat. No. 4,501,646 to W. G. Herbert.
A process for electroforming hollow nickel articles having a large crosssectional area onto a mandrel is described in U.S. Pat. No. 3,844,906 to R. E. Bailey et al. More specifically, the process involves establishing an electroforming zone comprising a nickel anode and a cathode comprising a support mandrel, the anode and cathode being separated by a nickel sulfamate solution maintained at a temperature of from about 140.degree. F. (60.degree. C.) to 150.degree. F. (66.degree. C.) and having a current density therein ranging from about 200 to 500 amps/ft.sup.2, imparting sufficient agitation to the solution to continuously expose the cathode to fresh solution, maintaining this solution within the zone at a stable equilibrium composition comprising:
Total Nickel: 12.0 to 15.0 oz/gal PA0 Halide as NiX.sub.2.6H.sub.2 O: 0.11 to 0.23 moles/gal PA0 H.sub.3 BO.sub.3 : 4.5 to 6.0 oz/gal
electrolytically removing metallic and organic impurities from the solution upon egress thereof from the electroforming zone, continuously charging to the solution about 1.0 to 2.0.times.10.sup.-4 moles of a stress reducing agent per mole of nickel electrolytically deposited from the solution, passing the solution through a filtering zone to remove any solid impurities therefrom, cooling the solution sufficiently to maintain the temperature within the electroforming zone upon recycle thereto at about 140.degree. F. (60.degree. C.) to 160.degree. F. (71.degree. C.) at the current density in the electroforming zone, and recycling the solution to the electroforming zone. The thin flexible endless nickel belt formed by this electrolytic process is recovered by cooling the nickel coated mandrel to effect parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion.
For metal articles fabricated by electroforming on mandrels having a small cross-sectional area, the process described in U.S. Pat. No. 4,501,646 to W. G. Herbert is preferred to overcome difficulties in removing the electroformed article from the mandrel. For example, when the chromium coated aluminum mandrel described in U.S. Pat. No. 3,844,906 is fabricated into electroforming mandrels having very small diameters of less than about 1 inch, metal articles electroformed on these very small diameter mandrels are extremely difficult or even impossible to remove from the mandrel. Attempts to remove the electroformed article can result in destruction or damage to the mandrel or the electroformed article, e.g. due to bending, scratching or denting.
Although electroforming techniques provide excellent hollow metal articles these processes exhibit certain deficiencies. Normally, hollow electroformed articles such as metal tubes or belts are removed from one end of an electroforming mandrel. Each end of these electroformed articles are usually rough and irregular and must be finished by trimming, for cosmetic reasons or to satisfy tolerance requirements. However, trimming the edges of electroformed articles by cutting blades, lasers, or turning on a lathe produce relatively rough edges or sharp edges which often must be coated to blunt the edge. Such trimming steps are undesirable in many commerical applications. When metal articles fabricated by electroforming on mandrels having a small cross-sectional area such as electroformed tubes are to be utilized as shafts, the ends of the tubes must normally be fitted with collets, press fit bearings or other devices which will allow the ends of the shaft to be supported by rods, bearings and the like. The additional cost, difficulty and manufacturing steps required to trim the ends of the electroformed articles prior to insertion of collets, bearings, or other support devices are highly undesirable, particulary when the electroformed tubes have a small diameter opening.
One well known alternative to trimming the ends of an electroformed tube is to mask the electroforming surface of the mandrel to prevent deposition of metal during the electroforming process. However, masking also requires an additional manufacturing operation. Moreover, electroforming masks have a short life and generally adhere poorly to an electroforming surface, particularly when the masking material has a different coefficient of expansion than the electroforming surface. In addition, many mask materials tend to absorb plating bath material and become electrically conductive thereby defeating the function of the mask. Also, the masks are difficult to apply. Often, electroformed metal deposits adjacent masked areas are rough and become progressively rougher as the mask ages. Also, the mask material may smear onto other parts of the mandrel during removal of the electroformed part and cause non-uniform nucleation and roughness of subsequently deposited electroformed articles.
One technique for masking is disclosed, for example, in U.S. Pat. No. 3,830,710 to Narozanski et al in which a flat masked cathode in an electrolytic deposition process of copper is described. Referring to FIG. 3 of the patent, masking member 24 is dove-tailed in shape and is adapted to mate with adjacent edge portions 14 of a flat cathode 10 to produce a smooth-edged surface adjacent the masking member. A "V-groove 17" is also described which causes the copper to deposit in the form of dendrites which grow in directions normal to the sides of the V-groove so that where the dendrites meet in the course of their growth, a plane of weakness is established. The deposited copper sheet fails at the plane of weakness when it is stripped from the flat electrode surface. The process described in this patent utilizes a flat unitary cathode to form copper sheets. The masking member appears to at least occasionally encounter leaks where copper deposits under the mask. The conductive deposits in the V-groove would be difficult to remove for cleaning. In addition, insulating dirt depositing in the groove may function as a mask. Further, it appears that removal of electrodeposited metal on the flat electrode surface requires peeling off of the electroformed material because the electrodeposited material cannot be slid off the end of the flat electrode surface.
In U.S. Pat. No. 3,022,230 to Fialkoff, a masking agent 2 is employed on a conductive mandrel to produce a helical groove 4 on mandrel 1. The usual problems encountered with masks as described above would also be expected using the technique of this patent.
In U.S. Pat. No. 799,634, to Cowper-Coles a cylindrical mandrel contains a fine spiral groove or indentation to allow deposited metal to be wound off the mandrel. Referring to FIG. 5, the metal deposited can be stripped off in a continuous spiral after the required thickness of metal has been deposited upon the mandrel. The mandrel described in this patent utilizes a unitary mandrel to form strips, wires and rods. Moreover, conductive deposits in the groove would be difficult to remove for cleaning. In addition, insulating dirt depositing in the groove may function as a mask. Further, it appears that removal of electrodeposited metal on a cylindrical mandrel requires unwinding of the electroformed material because the feathered extension of the electrodeposited material into the helical groove will prevent sliding of the material off the end of the cylindrical mandrel.